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Detonation?!?

Bolisk

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441
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Crystal Lake IL
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1972 LS5 Convertible PS, PB, A/C
A bit of a story, so bear with me.

My 1970 L46 (350/250) came from the factory with 11:1 compression. The previous owner swapped all the internal parts for correct 1970 LT-1 parts when he had it rebuilt. During the rebuilt, the engine was bored .030 over, and he put in TRW forged pistons that maintain the correct 11:1 mechanical ration. He did however have the intake and exhaust valves clearanced to provide better air flow (so the theory goes), so i'm guessing the dynamic compression would be a bit less than stock due to more volume in the chamber. He ran the car on premium pump gas (i'm guessing 93) for years without having any issues with an initial timing of 12 degrees (or so he claimed).

After purchasing the car, and learning/liking to drive it harder and harder. I noticed a noise (like something was loose) in the engine compartment when I'd get on in under load while already moving. Like trying to pass someone on the highway. The engine would dog slightly. . .and there would be this sound (I wouldn't call it a "ping") but a sound like something was loose in the exhaust in the engine compartment. This would only happen on really hot days (over 90 degrees) and after I've been driving for a few hours.

Eventually, I burned out the exhaust valves and had to get them replaced with hardened valve seats. (BTW: lead substitutes did nothing for my engine. . .and I was religious about it). The heads were milled (I have no idea by how much. . .because the machine shop forgot to write it down) and I put a stock .041 gasket in place. . .and went about driving the car again. Noticed however that this time the noise was happening seemingly more often. . .and didn't need as much RPM or loading the car for it to occur. Since no one I know has ever heard detonation. . .hard to get a second opinion.

So I reduced the timing advance to 10 on some advice. . .but not sure if it made much difference. . .I cannot remember.

Then one of my rockers failed, and I replace all my rockers with Comp Cams Magnum roller tip rockers which are said to get .040 of more lift because they don't flex like stock stamped rockers. Almost immediately I noticed that the noise now occurs whenever I get above 3500 RPM under hard acceleration. . .and every time it get on it while already moving. Needless to say, when it happens I immediately take my foot off the gas. I'm guessing this is because there is more air fuel to be compressed with the new rocker lift.

I do not notice any loss of power. . .or anything other than this noise.

I've noticed that all of the old school guys who would be able to identify this noise, who have been working in shops for years. . .are almost completely def. . .and cannot hear the noise I hear. . .so I'm on my own on this one.

So yesterday. . .I took the car out and confirmed that it was making the noise as I drove out of my way to a gas station that sells 100 octane race fuel (sonoco GT 100). I had just shy of 4 gallons of space in my tank. . .so I topped it off with 100. By my calculation (using online mix calculators) I had 16 gallons of 93, and 4 of 100. . .with a mixed octane rating of 94.4.

I drove the car for about five minutes to make sure I had the now mixed fuel in my carb. . .and I got on it. Surprisingly I could not get the car to make the noise unless under load and VERY high RPM's, just shy of read line. So the situation got better.

So I now believe this to be detonation of some type. . .and figure I must be close to the bear minimum octane rating for my car to work I figure 95 or 96 would be great. The problem is this race fuel is WAY too expensive at $8 a gallon.

As such, I would like to know if there is something I can to do improve the situation without dumping more money into fuel all the time.

I've read that move from .041 to .060 head gaskets can lower the compression by 1/4 point. Would that be enough to run on 93?

I know that I can switch back to stock rockers. . .but I spent so damn much on this comp cam ones. ..it just don't really want to throw that money spent down the drain. . .if I can help it. So I'm looking at other alternatives.

I'm still trying to understand why retarding the timing more might help. . .as everything I have read suggests that detonation is something that happens after spark occurs. . . usually by another source. . .etc. ..etc. .

Advice? Input? Options?

Thanks in advance!
 

Bolisk

Well-known member
Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
I've read other posts from JohnZ and Hib saying that 11:1 on 93 is possible without detonation. . .and I would have agreed for 4 of the 5 years of my ownership. Now i'm getting issues. . .and I'm not sure of the cause.
 

Vettehead Mikey

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Not that far from Ottawa
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1973 Coupe
Detonation is always 'post-spark' and is completely spontaneous in nature as a function of heat and pressure causig the fuel to explode instead of burn. Eliminating detonation by retarding the timing is accomplished by having the normal conflagration flame initiated by the spark reach and consume the unburned gasses in the combustion chamber prior to the heat and pressure causing them to spontaneously combust as mentioned above.

You might want to verify the timing curve before making any other mods. It could be that you've got too much advance at higher RPM. Masking that with higher octane is a mistake and lowering the compression would be a waste.
 

Peer81

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2,497
Location
Netherlands
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'81 Black
What I would do...
First suck out all the fuel and put it in an other car you have. Second put in 100 octane fuel from bottom to top then drive it.
By then you know for sure if detonation is the problem I would say 93 ron or 94.4 ron isn't that great to compare.

Then IF you want to change the headgasket you can go the extra mile but knowing how low the pistons are in the hole and you can cc the heads. Btw. SCR doesn't say much to me it's the DCR that is important! So you have to know which cam is in the block.

Greetings Peter
 

Bolisk

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Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
It's the stock lt-1 cam. Cranking psi is close to 220 per cylinder.
 

Vettehead Mikey

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Messages
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Not that far from Ottawa
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1973 Coupe
What I would do...
First suck out all the fuel and put it in an other car you have. Second put in 100 octane fuel from bottom to top then drive it.
By then you know for sure if detonation is the problem I would say 93 ron or 94.4 ron isn't that great to compare.

Peter-

our fuel here is never rated on the RON scale, it's all on the AKI scale. Our 93 AKI is your 98 RON.
 

Hib Halverson

Technical Writer for Internet & Print Media
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71 04 12 19
I've read other posts from JohnZ and Hib saying that 11:1 on 93 is possible without detonation. . .and I would have agreed for 4 of the 5 years of my ownership. Now i'm getting issues. . .and I'm not sure of the cause.

I've never said 11:1 CR on 93 is possible in a Gen 1 SBV8 with cast iron heads. If you're going to stick with 11:1 and the proper spark curve, you need around 96-oct r+m/2

Your compression ratio is too high. You either need to change pistons or go to heads with larger chambers.

Iron head performance engines with camshaft profiles typical of high-performance street engines run best on pump gas (91-93-oct) with between 9.75:1 and 10.0:1. If you got to aluminum heads, you can go up half a ratio.

Yes, you can eliminate detonation by retarding spark, but you end up with an engine that has poor response, runs hot and get crappy mileage.
 

Bolisk

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Joined
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Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
Ok, sorry for the miss quote. I must have miss read that.

I guess I'm confused as to why the problem is getting worse over time. It didn't do this when I first bought the car. Could the distributor be wearing? Springs weakening. . .vaccum advance going wrong? I'm planning on getting the distributor re-curved.

Can putting a think head gasket help?

BTW: this car does not run hot. It actually runs cooler than I would like.
 
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Hib Halverson

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71 04 12 19
Ok, sorry for the miss quote. I must have miss read that.

I guess I'm confused as to why the problem is getting worse over time. It didn't do this when I first bought the car. Could the distributor be wearing? Springs weakening. . .vaccum advance going wrong?

Can putting a think head gasket help?

If you think the problem is getting worse over time, it may be that carbon deposits are building on the chamber walls and the piston top.

You asked if the distributor could be wearing so you must not know how the distributor is curved. What is the total advance? How much of that is centrifugal? Is the advance limit bushing still in place?

A "thick" head gasket will mitigate the issue but will not eliminate the problem.

To correctly address this issue, you need to lower your compression 1-1.5 points....way more than you can get with a head gasket change. Actually, I remember you said earlier the heads had been milled so you may be worse off than you know.

If it were me, I'd be thinking about 10.0:1 with modern aluminum heads and a hyd. roller camshaft. An engine like that will make more power than the early L46 or LT1.
 

Bolisk

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Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
Yeah I don't know. I will pull the cap tommrow and have a look. I want to have it recurred by come one with a machine, just to be sure.

Seemed to get much worse with the new rockers. The comp cam magnums.

I really want to try to keep it original if possible. Plus have a 72 454 I'm working on and need the money for that. So new heads are tough to swallow.

Psi is 220. Is that high?
 
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Bolisk

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Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
I should add that I ran a quick test tonight. I reached into the ignition cover and man handled the distributor over and added about 1/8 turn advance. Just to see if it made a difference. I could not get the car over 1500 rpm wihtout the noise happening. Then I man handled the distributor in the other direction 1/4 turn. Problem solved. I was in a parking lot. I have no idea what that advance is now at. . .but I was able to stop the problem. I suspect that the advance it now so low that it's pointless.

anyone know how much material is removed in a normal head milling?
 
Joined
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Rio Rancho, NM
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1981 HD Suspension; ZN1 Option
YES!!!!! Bud not nearly as loud. And only happens when I get on it under load!!

Ok. What I found it happened in my Vette was that it "jump" cogs between the cam and the distributor gear.

So, find TDC for your Vette's #1 cylinder, and check the location of the vibration/torsional dampener TDC mark to the timing tab and the relative location of distributor's rotor to the #1 electrode on the distributor cap.

You'll have to check for a sheared distributor pin or maybe the distributor gear cogs are worn. The distributor gear is replaceable. But at least, a jump on the cogs took place somehow. Also, check for broken flyweights springs in distributor. With a broken spring, it will automatically advance total timing at idle; however, the sign of this should be a higher curb speed on the engine...not backfiring like it is doing.


That video depicts the car making that noise under load as well. That road I was on is I-40 east, which west of Albuquerque, it has a series of approximately 5% grade, and the Vette was having a hard time climbing. My speed was 45 climbing the hills. :mad :( :D
 
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Hib Halverson

Technical Writer for Internet & Print Media
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Messages
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CenCoast CA
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71 04 12 19
Yeah I don't know. I will pull the cap tommrow and have a look. I want to have it recurred by come one with a machine, just to be sure.

Seemed to get much worse with the new rockers. The comp cam magnums.

I really want to try to keep it original if possible. Plus have a 72 454 I'm working on and need the money for that. So new heads are tough to swallow.

Psi is 220. Is that high?

Provided your distributor advance limit bushing is still in place and the centrifugal advance mechanism is working properly, given you can't "swallow" new heads, then you other choices are either 1) tear the engine down and change pistons or 2) Use the thickest head gasket you can get along with a slow spark curve, reduced total advance and good cooling to keep the engine out of detonation. If you choose #2, accept the fact that throttle response, performance and mileage will suffer.

As suggested by others...

1) Set the engine at TDC #1 by lining up the timing mark on the damper with the 0 index on the timing cover.
2) Mark the dist. body even with the #1 spark plug wire socket
3) Remove the cap and verify the rotor points to your mark. If it does, the engine is timed close enough for now. Go to #4. If it doesn't, set the engine at TDC #1 using the positive stop method, then check for a slipped outer balancer ring, a faulty timing chain or a distributor which has been installed a tooth off.
4) Inspect the advance mechanism. Repair/replace any faulty parts
5) Have the dist. curve set for 30-32 deg and full advance coming late.
6) Limit vacuum advance to 8-10 deg.

As for the Comp rockers, I wouldn't worry about them right now. Get the engine running right with stock rockers, then swap the Comp 1.6es back in. There's only a modest performance gain with them and your stock rockers do not flex.
 
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Bolisk

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Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
I'm using the Lectriclimted Breakerless SE electronic ignition. . .no points, so dwel should not be an issue. I suppose that unit could be toast. . .but I doubt it.

Car runs cool. . .never gets above 200 even on the hottest days.

I'll pull the distributor and check it over and for proper placement. . .and see if that makes a difference. It's possible. I had the intake manifold off and on a million times when I thought I had a oil leak. Turned out to be valve guide seals. . .so I may have put the distributor on wrong the last time. Good point to check.

is the mechanical advance limit stop something obvious on the top of the distributor? I just want to make sure what I'm looking for.

I would think that the amount of material they removed in the heads for "clearance-ing" the valves by the previous owner would be enough to offset the loss in head height. . .to be below 11:1. . .but maybe I'm wrong. I was thinking a .060 or .080 head gasket might get me back to where I was when I first bought the car. The noise sounds EXACTLY like the one in the video link that was posted. Just not as loud. Is that noise detonation?
 

Hib Halverson

Technical Writer for Internet & Print Media
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Messages
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71 04 12 19
The advance limit bushing is what controls how long the spark curve is. Once you have the distributor out, the cap off and the weights and springs off, you'll see that the movement of the distributor cam is limited by a pin which has a bushing pressed over it. That bushing is either nylon or bronze. That bushing is what limits the advance.

You need to verify that bushing is in place.

SCE has some .060 composite racing head gaskets for Gen 1 SBV8s.
 

Vettehead Mikey

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Messages
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Not that far from Ottawa
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1973 Coupe
I'll pull the distributor and check it over and for proper placement. .

If you can rotate the housing to achieve the desired timing, it is in the proper place. The one tooth off thing is only if the housing cannot be rotated sufficiently to be able to time properly.

Is that noise detonation?

No. Detonation sounds like marbles in a coffee can, not a rhythmic clatter like in the video.
 

Bolisk

Well-known member
Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
The advance limit bushing is what controls how long the spark curve is. Once you have the distributor out, the cap off and the weights and springs off, you'll see that the movement of the distributor cam is limited by a pin which has a bushing pressed over it. That bushing is either nylon or bronze. That bushing is what limits the advance.

You need to verify that bushing is in place.

SCE has some .060 composite racing head gaskets for Gen 1 SBV8s.


I saw some at summit racing. ..but I thought they were all copper. I will look for the composite. Wheres the best place for SCE?
 

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